Adults with mastocytosis reportedly have poor attention span, irritability, difficulty in concentration, forgetfulness, confusion, mood fluctuation, and depression thought to be due to excess histamine. We have assessed 12 children with mastocytosis to determine if similar behavioral pathology is evident. Results of this study indicate no clear excess of pathology exists in children with mastocytosis. Individual children treated with antihistamines have a nonspecific increase in behavioral difficulties at rates similar to other medically ill groups. No unique behavioral pattern implicating histamine overproduction was identified. We participated in a multi-center consensus conference in 1990 to develop a uniform classification for mastocytosis and uniform evaluation. The results of this conference relied heavily on data obtained at NIH. These consensus recommendations are available and will result in the standard approach to the diagnosis and treatment of this disease. We have completed a study to characterize liver disease in 40 patients with mastocytosis. All patients had routine liver function tests, liver- spleen scan, and liver biopsy, if indicated. We are in the process of reviewing this data to answer the following question: 1. What is the prevalence of liver disease in this population? 2. What are the characteristic features of this liver disease? 3. What are the indications for liver biopsy? 4. Is there a relationship between liver disease and mastocytosis disease category?